This study was undertaken to determine the mechanisms controlling the bronchial circulation with the ultimate goal of determining the relationship of bronchial blood flow to such tracheobronchial functions as mucus secretion, ciliary clearance, and heat and water exchange with inspired air. It has been shown that about 85 percent of the intrapulmonary bronchial blood flow in the sheep was delivered by a single artery large enough to permit the use of an electromagnetic flowmeter to measure blood flow. Under baseline conditions in chloralose anesthetized sheep, bronchial blood flows were recorded equal to 0.1 to 0.6 percent of cardiac output. It was also found that the bronchial circulation does not respond to arterial occlusion with the classical reactive hyperemia seen in many other systemic vascular beds. In the coming year other locally mediated responses will be investigated such as the responses to variations in perfusion pressure (autoregulation), hypoxemia, and temperature. Because of the anastomoses between the bronchial and pulmonary circulations, the hemodynamic responses to varying right and left atrial pressures and aortic pressure will be studied. It was also found that raising tracheal pressure from 5.0 to 30.0 cm H20 caused 50 percent increases in bronchial vascular resistance. Additional experiments are to be conducted to determine the mechanism of this response and how it relates to the clinical use of positive end expiratory pressure.